My younger brother stopped around 4 or maybe 5 but it is different for every child. If they still wet the bed then leave the nappies. Make sure your child gets into a habit of going to the restroom before bed every night so he won't need to wear one.
Whenever the child is ready. Most all will quit wearing them in time. But there are a rare few like me that get an emotional attachment for diapers. Nothing wrong with that and it doesn't hurt anyone.